Problem: On the first 4 quizzes of her language class, Gabriela got an average score of 83. What does she need on the next quiz to have an overall average of 86?
Solution: Let her score on the next quiz be $x$ The sum of all of her scores is then $4 \cdot 83 + x$ The same sum must also be equal to $5 \cdot 86$ Solve: $x = 5 \cdot 86 - 4 \cdot 83 = 98$.